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580 J. Chem. Eng. Data 2007, 52, 580-585

Thermodynamic Properties of Three Pyridine Carboxylic Acid Methyl Ester Isomers

Maria D. M. C. Ribeiro da Silva,* Vera L. S. Freitas, Luı´s M. N. B. F. Santos, Michal Fulem, M. J. Sottomayor, Manuel J. S. Monte, and W. E. Acree, Jr. Centro de Investigac¸a˜o em Quı´mica, Departamento de Quı´mica, Faculdade de Cieˆncias, Universidade do Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal

The present work reports the values of the standard (p° ) 0.1 MPa) molar energies of combustion for the isomers methyl picolinate and methyl isonicotinate and for the crystalline isomer methyl nicotinate measured by static bomb calorimetry. The molar of of the three isomers at T ) 298.15 K as well as the molar enthalpies of sublimation of two of them (methyl picolinate and methyl nicotinate) were derived from measurements at different using a static method. The molar enthalpies of vaporization at T ) 298.15 K of methyl picolinate and methyl isonicotinate were also measured using Calvet microcalorimetry. The thermal behavior of the compounds was studied by differential scanning calorimetry, and the temperatures and the molar enthalpies of fusion were determined using this technique. The experimental results were used to derive the gaseous standard molar enthalpies of formation, at 298.15 K, of the three pyridinecarboxylic acid methyl ester isomers and the triple points of the ortho and meta isomers.

Introduction Thermochemical data are essential for the understanding of the relations between the structure and reactivity of molecules, and that reason justifies the importance of the study of key compounds from several classes of compounds because such data permit clarification of the chemical behavior of the corresponding species. In this context, systematic experimental and theoretical thermochemical studies on heterocyclic mol- Figure 1. Molecular formulas for pyridinecarboxylic acid methyl ester ecules derived from pyridine have been done in our Thermo- isomers: (a) methyl picolinate, (b) methyl nicotinate, and (c) methyl chemistry Laboratory for more than a decade in order to evaluate isonicotinate the energetic effects of different substituents on the pyridinic ring.1-14 The main goal of the present work is to determine the gaseous enthalpies of formation of methyl picolinate (2- 3-PyCOOCH3 (CASRN 93-60-7) and 4-PyCOOCH3 (CASRN PyCOOCH3), methyl nicotinate (3-PyCOOCH3), and methyl 2459-09-8) were prepared by the esterification of nicotinic acid isonicotinate (4-PyCOOCH3), represented in Figure 1, together and isonicotinic acid, respectively, with trimethylorthoacetate with the measurement of other relevant properties to the according to the published synthetic method of Trujillo and thermodynamic characterization of those isomers. Gopalan.15 Both products were collected and further purified The present work reports for the three pyridinecarboxylic acid by fractional distillation under reduced pressure. The purities methyl ester isomers the experimental values of the standard of 2-PyCOOCH3 and 4-PyCOOCH3 were checked by GLC molar energies of combustion, the standard molar enthalpies of (using an HP 4890 apparatus with a 15 m length column HP-5, vaporization, and the standard molar enthalpies of sublimation 5 % diphenyl, and 95 % dimethylpolysiloxane under a nitrogen s of two of them methyl picolinate and methyl nicotinate. For pressure of 21 kPa; the oven was 423 K), whereas methyl isonicotinate, the of sublimation was estimated the purity of 3-PyCOOCH3 was confirmed by DSC after it has from the enthalpies of fusion and of vaporization. been dried at T ) 303 K under reduced pressure. For The experimental results are used to derive the gaseous 3-PyCOOCH and for 4-PyCOOCH , microanalytical determi- ) 3 3 standard molar enthalpies of formation, at T 298.15 K, of nations yielded the following results (mass fractions). For the three pyridinecarboxylic acid methyl ester isomers. In order 3-PyCOOCH3:C7H7NO2, C, 0.6139; H, 0.0516; N, 0.1011. For to complement the results, the thermal behavior of the com- 4-PyCOOCH :CH NO , C, 0.6137; H, 0.0507; N, 0.1013. pounds was studied, and the temperatures and the molar 3 7 7 2 Calcd C, 0.6131; H, 0.0514; N, 0.1021. The percentage of CO enthalpies of fusion were determined by differential scanning 2 recovered in the combustion experiments also showed the high calorimetry. purity of the samples. Experimental Section The densities (F) of the samples and the average ratios (µ) Materials. The compound 2-PyCOOCH3 (CASRN 2459-07- of the mass of recovered to that calculated from 6) was a commercial sample from Aldrich, 99 %. The isomers the mass of sample, together with the standard deviation of the F) ‚ -3 16 * Corresponding author. E-mail: [email protected]. Phone: +351 22 6082 mean, were as follows: 2-PyCOOCH3, 1.137 g cm , µ -3 838. Fax: +351 22 6082 822. ) (0.9999 ( 0.0003); 3-PyCOOCH3, F)1,0 g‚cm (assumed), 10.1021/je060472e CCC: $37.00 © 2007 American Chemical Society Published on Web 02/06/2007 Journal of Chemical and Engineering Data, Vol. 52, No. 2, 2007 581

-3 16 28 µ ) (0.9998 ( 0.0003); 4-PyCOOCH3, F)1.161 g‚cm , µ standard molar enthalpies of sublimation of and ) (0.9980 ( 0.0005). of vaporization of n-decane.29 The determination of the enthalpy of sublimation of the 3-PyCOOCH3 isomer has not been Calorimetric Measurements performed using Calvet microcalorimetry because its low temperature of fusion and high vapor pressure did not allow Static Bomb Calorimetry. The energies of combustion of the the accurate use of this technique. three compounds were determined using an isoperibol static bomb calorimeter, with a twin valve bomb of internal volume Differential Scanning Calorimetry. Enthalpies and temper- 0.290 dm3. The calorimeter and technique have been previously atures of fusion of the three isomers were determined with a described.17-19 The energy equivalent of the calorimeter was Setaram DSC 141 calorimeter. Thermograms of samples determined twice during this experimental study, by the hermetically sealed in steel pans were recorded in an air · -1 combustion in oxygen of thermochemical standard benzoic acid atmosphere at a scanning rate of 0.017 K s . All the pans were weighed before and after the experiments to confirm that no NBS, SRM 39j, with the massic energy of combustion of ∆cu )-(26434 ( 3) J‚g-1 (value under certificate conditions). The product leakage had occurred. A Mettler UMT2 microbalance, × -7 calibration results were corrected to give the energy equivalent with a sensitivity of (1 10 ) g, has been used. (cal) corresponding to the average mass of water added to the For each compound, at least five samples weighing (8 to 10) calorimeter, 2900.0 g. For the study of isomers 3-PyCOOCH3 mg were recorded. To verify the stability of each compound and 4-PyCOOCH3, one set of 12 calibration experiments was after , several scans were performed for each sample. made in oxygen at p ) 3.04 MPa with 1.00 cm3 of water added Both the cooling and heating scans were reproducible after -1 to the bomb and yielded cal ) (15546.5 ( 1.1) J‚K , where several cycles for the three isomers. The samples were studied the uncertainty quoted is the standard deviation of the mean. over the following temperature ranges: (253 to 303) K for For the study of 2-PyCOOCH3, another set of 11 calibration 2-PyCOOCH3, (293 to 323) K for 3-PyCOOCH3, and (263 to -1 - experiments was made, yielding cal ) (15553.3 ( 0.9) J‚K . 303) K for 4-PyCOOCH3. No solid- transitions were For the combustion experiments, crystalline 3-PyCOOCH3 was observed for the three isomers over these temperature intervals. burnt in pellet form whereas the 2-PyCOOCH3 and The cooling scans from above the melting temperature of 4-PyCOOCH3 were enclosed in melinex bags, according to the each compound toward the lower temperature limits were technique described by Skinner and Snelson.20 The massic achieved using circulating water from a thermostatic bath for ° energy of combustion of dry melinex, ∆cu )-(22902 ( 5) the study of 3-PyCOOCH3 and using circulating dry air, -1 20 J‚g , was confirmed by combustion of melinex samples in refrigerated with liquid nitrogen, for the study of 2-PyCOOCH3 our laboratory. The mass of melinex used in each experiment and of 4-PyCOOCH3. In both cases, an approximately constant was corrected for the mass fraction of water (0.0032).20 All the cooling rate of 0.017 K·s-1 was attained. Each melting curve samples were ignited at T ) (298.150 ( 0.001) K in oxygen at allowed computing the temperature and of p ) 3.04 MPa, with 1.00 cm3 of deionized water previously the samples. The relative atomic masses used throughout this added to the bomb. paper were those recommended by the IUPAC Commission.30 The electrical energy for ignition ∆U(ign.) was determined Vapor Pressure Measurements. The vapor of the from the change in potential difference across a 1400 µF three isomers were measured at different temperatures using a capacitor when discharged through the platinum ignition wire. static apparatus based on a capacitance diaphragm gage. The For the cotton thread fuse, of empirical formula CH1.686O0.843, apparatus and the measuring procedure have been recently -1 21 31 the massic energy of combustion is ∆cu° )-16250 J‚g . described in detail, so only a short description is given here. The corrections for nitric acid formation, ∆U(HNO3), were It is constructed of stainless steel tubing of internal diameter based on the value -59.7 kJ‚mol-1,22 for the molar energy of 17 mm with connections ConFlat DN 16 CF and includes all -3 formation of 0.1 mol‚dm HNO3(aq) from N2,O2, and H2O- metal angle valves, VAT series 57 high-temperature range for (l). An estimated pressure coefficient of specific energy: (∂ u/∂ UHV, operated pneumatically. The pressure is measured by a -1 -1 p)T )-0.2 J‚g ‚MPa at T ) 298.15 K, a typical value for capacitance diaphragm absolute gage MKS Baratron 631A01- most organic compounds,23 was assumed. The mass of com- TBEH, with a measuring upper limit of 133 Pa and the pound, m(compound), used in each experiment was determined uncertainty of 0.25 % of the reading pressure, as stated by the from the total mass of carbon dioxide, m(CO2, total), produced manufacturer. The temperature of the pressure sensor is kept at after allowance for that formed from the cotton thread fuse and T ) 423 K by the self-controlling temperature system. The melinex. For each compound, the standard massic energy of pressure gage has been calibrated at 423 K by the manufacturer combustion (∆cu°) was calculated by the procedure given by at seven equally spaced pressures from 0 to 133 Pa with a Hubbard et al.24 maximum deviation of 0.23 %. This calibration is traceable to CalWet Microcalorimetry. The enthalpies of vaporization of the National Institute of Standards and Technology (NIST). 2-PyCOOCH3 and 4-PyCOOCH3 were determined in a Calvet The temperature of the sample is measured using a platinum high-temperature microcalorimeter (SETARAM HT 1000), resistance thermometer Pt100 class 1/10 (in a four-wire con- using the “vacuum-sublimation drop microcalorimetric method”,25 nection), which is in a good thermal contact with the sample. adapted for liquids.26 Samples about (6 to 9) mg of the liquid This thermometer was calibrated by comparison with a SPRT compounds, contained in a thin capillary tube sealed at (25 Ω; Tinsley, 5187A). The uncertainty of the temperature one end, were dropped at room temperature into the hot reaction measurements is estimated to be better than ( 0.01 K. All vessel of a high-temperature Calvet microcalorimeter held at T temperatures reported here are based on the international ) 386 K for 2-PyCOOCH3 or at T ) 388 K for 4-PyCOOCH3 temperature scale ITS-90. and then removed from the hot zone by vacuum . The apparatus has been tested by measurements of vapor g,T o The observed enthalpies of vaporization, ∆l,298.15KHm, were pressure of recommended reference materials (naphthalene, ) T o corrected to T 298.15 K, using ∆298.15KHm(g) estimated by a benzoic acid, benzophenone, and ferrocene) and proved to group method based on the values of Stull et al.27 The provide reliable vapor pressure data as well as enthalpies of microcalorimeter was calibrated in situ using the reported sublimation or vaporization. The uncertainty in the pressure 582 Journal of Chemical and Engineering Data, Vol. 52, No. 2, 2007

Table 1. Typical Combustion Experiments at T ) 298.15 Ka Table 3. Derived Standard (p° ) 0.1 MPa) Molar Values in the Liquid or Crystalline Phases, at T ) 298.15 Ka 2-PyCOOCH3 3-PyCOOCH3 4-PyCOOCH3 ° ° ° compound ∆cUm(cr, l) ∆cHm(cr, l) ∆fHm(cr, l) m(CO2, total)/g 1.81149 1.40994 1.96846 m(compound)/g 0.75391 0.62594 0.81060 2-PyCOOCH3(l) -3481.5 ( 1.6 -3482.1 ( 1.6 -272.9 ( 1.8 m(melinex)/g 0.05002 0.06263 3-PyCOOCH3(cr) -3460.1 ( 1.8 -3460.7 ( 1.8 -294.2 ( 2.0 m(fuse)/g 0.00206 0.00236 0.00252 4-PyCOOCH3(l) -3476.0 ( 1.5 -3476.6 ( 1.5 -278.3 ( 1.8 ∆Tad/K 1.30925 1.02071 1.41952 -1 a -1 f/(J·K ) 14.50 14.20 14.72 Values in kJ‚mol . ∆m(H2O)/g -0.8 -0.3 -2.1 -∆U(IBP)b/J 20377.73 15881.65 22076.96 Table 4. Calorimetric Values for the Standard (p° ) 0.1 MPa) -∆U(Melinex)/J 1145.66 1434.42 Molar Enthalpies of Vaporization, at T ) 298.15 K -∆U(fuse)/J 33.45 38.33 40.92 T ∆g,T H° ∆T H°(g) ∆gH° (298.15 K) -∆U(HNO3)/J 42.17 31.76 37.13 no. of l,298.15K m 298.15K m l m ∆U(ign.)/J 0.67 1.19 1.18 compound expts K kJ·mol-1 kJ·mol-1 kJ·mol-1 - ∆UΣ/J 15.57 11.86 17.23 ( ( -1 2-PyCOOCH3 6 386 78.3 1.8 11.3 67.0 1.8 -∆cu°/(J·g ) 25387.88 25239.59 25346.81 4- PyCOOCH3 7 388 79.2 ( 1.4 13.8 65.4 ( 1.4 a m(CO2, total) is the total mass of CO2 formed in the experiment. Table 5. Temperatures and Enthalpies of Fusion of 2-PyCOOCH , m(compound) is the mass of compound burnt in the experiment. m(fuse) is 3 3-PyCOOCH3, and 4-PyCOOCH3 the mass of fuse (cotton) used in the experiment. ∆Tad is the corrected temperature rise. f is the energy equivalent of contents in the final state. indirect method ∆m(H2O) is the deviation of the mass of water added to the calorimeter calorimetric (vapor pressures) from 2900.0 g. ∆U(IBP) is the energy change for the isothermal combustion T ∆l H (T ) ∆l H (T ) T p reaction under actual bomb conditions. ∆U(melinex) is the energy of fus cr m fus cr m Tr Tr Tr combustion of melinex. ∆U(fuse) is the energy of combustion of the fuse compound K kJ·mol-1 kJ·mol-1 KPa (cotton). ∆U(HNO ) is the energy correction for the nitric acid formation. 3 2-PyCOOCH 289.2 ( 0.2 16.8 ( 0.1 17.5 ( 0.3 293.4 3.68 ∆U(ign.) is the energy of ignition. ∆U is the energy correction to the 3 Σ 3-PyCOOCH 312.6 ( 0.1 19.3 ( 0.3 19.7 ( 0.2 312.7 56.4 standard state. ∆ u° is the standard massic energy of combustion. b ∆U(IBP) 3 c 4-PyCOOCH 283.4 ( 0.4 14.6 ( 0.3 includes ∆U(ign.). 3

Table 2. Individual Values of the Massic Energy of Combustion, normal thermochemical practice,32 the uncertainties assigned to ° ) ∆cu ,atT 298.15 K the standard molar enthalpies of combustion are, in each case,

2-PyCOOCH3 3-PyCOOCH3 4-PyCOOCH3 twice the overall standard deviation of the mean and include the uncertainties in calibration and in the values of auxiliary 25380.38 25221.54 25357.83 ° ° 25375.17 25216.61 25346.13 quantities. To derive ∆fHm(cr,l) from ∆cHm(cr,l) the standard 25376.34 25253.93 25324.35 molar enthalpies of formation of H2O(l) and of CO2(g) at T ) 25413.21 25218.28 25356.39 298.15 K, -(285.830 ( 0.042) kJ‚mol-1,33 and -(393.51 ( 25385.84 25235.93 25351.23 0.13) kJ‚mol-1,33 respectively, were used. Values of the standard 25387.88 25239.59 25346.81 g ° ) -1 molar enthalpies of vaporization, ∆l Hm,atT 298.15 K, -〈∆cu°〉/(J·g ) 25386.5 ( 5.7 25231.0 ( 6.0 25347.1 ( 5.0 measured using Calvet microcalorimetry, are given in Table 4, where the assigned uncertainties are twice the standard deviation measurements is adequately described by the equation σ(p/Pa) of the mean. ) 0.01 + 0.0025(p/Pa), and the workable temperature and The fusion temperatures were taken as DSC onset tempera- pressure ranges are (243 to 413) K and (0.4 to 133) Pa, tures. For each fusion peak, six integrations were performed in respectively. order to get the enthalpy of fusion. Table 5 displays the results obtained for each compound. There, the uncertainty associated Results and Discussion to each calorimetric result represents twice the standard deviation of the mean of at least six results. The results for a typical combustion experiment of each one The vapor pressures of 2-PyCOOCH and 3-PyCOOCH were of the three compounds are given in Table 1. In this table, 3 3 measured above the crystalline as well as above the liquid phase. ∆m(H O) is the deviation of the mass of water added to the 2 The of both liquids allowed measurements at the calorimeter from the mass assigned for (cal), 2900.0 g; ∆U Σ same temperature on both the crystalline and the metastable is the correction to the standard state;  is the energy of the f liquid phase over temperature intervals greater than 10 K. The bomb contents after ignition; ∆T is the temperature rise, ad vapor pressures of 4-PyCOOCH were measured only above corrected for exchange and the energy of stirring; and ∆U 3 ign. the liquid phase. The experimental results of the vapor pressures is the energy of ignition. The samples were ignited at T ) 298.15 listed in Tables 6 to 8 were fitted by the Clarke and Glew K, and the energy associated to the isothermal bomb process, 24 equation (eq 2): ∆U (IBP), was calculated from eq 1, where cp(H2O, l) is the heat capacity of liquid water: g 0 p ∆cdGm(θ) 1 1 R ln )- + ∆g H0 (θ) - + )-{ + + } + ( 0) cd m ( ) ∆U(IBP) cal ∆m(H2O)cp(H2O, l) f ∆Tad p θ θ T ∆U(ign.) (1) g 0 θ T ∆ C (θ) - 1 + ln (2) cd p,m [(T) (θ)] The individual results of all combustion experiments, together with the mean value and its standard deviation of the mean, where p is the vapor pressure, p0 is a selected reference pressure are given for each compound in Table 2. Table 3 lists the derived (in this work we took p0 ) 105 Pa), θ is a selected reference ° ) standard molar energies and enthalpies of combustion, ∆cUm temperature (in this work we took θ 298.15 K), R is the molar ° ) ‚ -1‚ -1 g 0 (cr,l) and ∆cHm(cr,l), respectively, and the standard molar constant (R 8.314472 J K mol ), ∆cdGm is the enthalpies of formation of the compounds in the condensed difference in molar Gibbs energy between the gaseous and the ° ) phase, ∆fHm(cr,l), at T 298.15 K. In accordance to the crystalline or liquid phases (condensed phase) at the selected Journal of Chemical and Engineering Data, Vol. 52, No. 2, 2007 583

a a Table 6. Experimental Data on Vapor Pressures of 2-PyCOOCH3 Table 8. Experimental Data on Vapor Pressures of 4-PyCOOCH3 T/K p/Pa ∆p/Pa T/K p/Pa ∆p/Pa T/K p/Pa ∆p/Pa T/K p/Pa ∆p/Pa Crystalline Liquid Liquid Liquid 278.20 0.582 0.001 293.12 3.570 -0.012 273.26 2.588 -0.019 298.13 24.27 -0.04 278.20 0.580 -0.001 293.13 3.566 -0.019 273.26 2.588 -0.019 298.14 24.28 -0.05 280.68 0.794 -0.003 298.15 5.628 0.038 275.74 3.349 0.016 300.63 29.60 -0.05 280.67 0.793 -0.003 298.15 5.601 0.011 275.74 3.323 -0.010 300.63 29.61 -0.04 283.16 1.086 0.000 303.15 8.522 -0.026 278.22 4.243 0.006 303.12 35.95 -0.04 283.15 1.087 0.002 303.15 8.506 -0.042 278.23 4.253 0.012 303.13 35.97 -0.05 285.65 1.476 0.003 308.08 12.80 0.01 280.71 5.373 0.010 305.62 43.47 -0.07 285.65 1.472 -0.001 308.11 12.79 -0.03 280.71 5.383 0.020 305.63 43.48 -0.10 288.14 1.988 0.001 313.12 19.19 0.19 283.19 6.753 0.006 308.09 52.19 -0.16 288.15 1.989 -0.001 313.13 19.16 0.15 283.19 6.753 0.006 308.09 52.19 -0.16 290.65 2.682 0.010 318.09 27.71 0.06 285.66 8.443 0.004 310.59 62.66 -0.18 290.63 2.681 0.015 318.09 27.63 -0.02 285.66 8.443 0.004 310.60 62.70 -0.19 288.16 10.54 0.01 313.07 74.95 -0.09 Liquid - 273.21 0.510 0.003 323.05 39.68 0.01 288.16 10.54 0.01 313.07 74.90 0.14 - 290.65 13.08 0.01 315.54 89.24 0.01 273.21 0.505 0.002 328.04 56.28 0.02 - 278.18 0.860 0.008 328.04 56.32 0.06 290.65 13.08 0.01 315.56 89.34 0.01 278.18 0.856 0.004 333.01 78.70 0.03 293.15 16.15 0.00 318.03 106.2 0.3 283.15 1.403 0.001 333.02 78.76 0.04 293.16 16.17 0.00 318.03 106.2 0.3 283.16 1.399 -0.004 337.99 108.8 0.0 295.65 19.90 0.02 320.52 125.6 0.4 288.13 2.242 -0.020 337.99 108.8 0.0 295.65 19.90 0.02 320.52 125.6 0.4 - - 288.14 2.246 0.018 340.47 127.0 0.2 a ∆p ) p - pcalc, where pcalc is calculated from the Clarke and Glew a equation with parameters given in Table 9. ∆p ) p - pcalc, where pcalc is calculated from the Clarke and Glew equation with parameters given in Table 9. Table 9. Sublimation and Vaporization Results at T ) 298.15 K 5 a and p° ) 10 Pa Table 7. Experimental Data on Vapor Pressures of 3-PyCOOCH3 g 0 g 0 g 0 T/K p/Pa ∆p/Pa T/K p/Pa ∆p/Pa ∆cdGm ∆cdHm ∆cdCp,m ‚ -1 ‚ -1 ‚ -1· -1 Crystalline Liquid compound phase J mol kJ mol J K mol a 273.23 0.651 0.001 298.14 18.19 0.03 2-PyCOOCH3 crystalline 23989 ( 7 81.79 ( 0.14 - 41 273.23 0.658 0.008 298.15 18.19 0.02 2-PyCOOCH3 liquid 24274 ( 3 64.06 ( 0.04 -86 ( 3 278.20 1.228 -0.002 303.13 27.15 -0.06 3-PyCOOCH crystalline 22253 ( 5 80.11 ( 0.13 -41 ( 16 - - 3 278.20 1.214 0.016 303.13 27.12 0.09 3-PyCOOCH3 liquid 21351 ( 4 61.2 ( 0.21 -107 ( 15 - b a 283.18 2.269 0.004 305.12 31.86 0.03 4-PyCOOCH3 crystalline 75.1 ( 0.3 - 41 283.19 2.282 0.006 305.13 31.88 0.03 4-PyCOOCH3 liquid 20626 ( 5 59.37 ( 0.02 -112 ( 3 283.19 2.271 -0.005 307.11 37.12 -0.01 - - 288.16 4.060 0.048 307.11 37.09 0.04 a ∆g C0 estimated according to the value derived for 3-PyCOOCH - - cd p,m 3 288.16 4.076 0.032 309.10 43.10 0.10 from eq 2 fitting. b Estimated from the above value of the enthalpy of - 293.16 7.318 0.036 309.10 43.06 0.14 vaporization and from the value of the enthalpy of fusion presented in Table 293.16 7.304 0.022 311.07 50.20 0.13 5. 298.14 12.70 0.08 311.08 50.06 -0.05 298.16 12.69 0.05 313.07 58.17 0.15 Table 10. Derived Standard (p° ) 0.1 MPa) Molar Enthalpies of 303.14 21.60 0.11 313.08 58.17 0.10 Formation, at T ) 298.15 Ka 303.14 21.59 0.10 315.06 67.22 0.19 ° ° ° 308.10 35.86 0.09 315.07 67.04 -0.04 compound ∆fHm(cr, l) ∆cr,lgHm ∆fHm(g) 308.11 35.87 0.06 317.04 77.07 -0.13 2-PyCOOCH3 (l) -272.9 ( 1.8 64.06 ( 0.04 -208.8 ( 1.8 310.60 45.66 -0.28 317.05 77.15 -0.11 3-PyCOOCH3 (cr) -294.2 ( 2.0 80.11 ( 0.13 -214.1 ( 2.0 310.61 45.95 -0.04 318.08 83.17 0.10 4-PyCOOCH3 (l) -278.3 ( 1.8 59.37 ( 0.02 -218.9 ( 1.8 310.61 45.57 -0.42 318.11 83.30 0.05 - 319.09 88.86 0.04 a Values in kJ‚mol-1. 319.09 88.86 -0.29 321.08 102.0 -0.3 321.08 103.0 0.7 adjusted for the temperature of fusion using the heat capacity 323.09 117.7 0.5 values presented in Table 9. The so derived values for the 323.09 117.7 0.5 enthalpies of fusion are in excellent agreement with the values 324.06 124.7 -0.4 324.06 124.6 -0.5 obtained by DSC attesting the reliability of the enthalpies of vaporization and of sublimation derived from the vapor pressure a ∆p ) p - pcalc, where pcalc is calculated from the Clarke and Glew measurements. So these values were selected instead of those equation with parameters given in Table 9. obtained from Calvet microcalorimetry for the of the ortho and para isomers, which are respec- reference pressure (the gaseous phase is supposed to have tively (3 and 6) kJ‚mol-1 higher than the values derived from 0 g 0 characteristics of ideal gas at the pressure p ), ∆cdHm is the vapor pressure measurements. The standard molar enthalpies difference in molar enthalpy between the gaseous and the of formation, in both the condensed and gaseous phases, and g 0 condensed phase, and ∆cdCp,m is the difference between the the selected values of the enthalpies of sublimation or vaporiza- heat capacities of the perfect gas and of the condensed phase. tion for the three compounds are summarized in Table 10. The parameters of the Clarke and Glew equation are presented The enthalpy of sublimation at the temperature of fusion of in Table 9. From the vapor pressure equations the temperature the para isomer was estimated from the enthalpy of fusion and the pressure of the triple points of the ortho and meta obtained from DSC measurements and from the value of the isomers were derived. These values are presented in Table 5 enthalpy of vaporization adjusted to the temperature of fusion, together with the values of the enthalpies of fusion derived from using the liquid heat capacity value presented in Table 9. The the values of enthalpies of vaporization and of sublimation enthalpy of sublimation at the temperature 298.15 K, presented 584 Journal of Chemical and Engineering Data, Vol. 52, No. 2, 2007

Table 11. Comparison of Values of Enthalpy of Formation, in the similar increment for the methylation of 4-pyridine carboxylic Gaseous State, at T ) 298.15 K, Measured in This Work with acid and to a higher increment for the methylation of 2-pyridine Literature Values carboxylic acid. This should reflect the effect of the intramo- ° · -1 · -1 compound ∆fHm(g)/kJ mol X increment/kJ mol lecular hydrogen bond in 2-pyridine carboxylic acid described Py 140.4 ( 0.734 by Shahawy et al.36 on the basis of theoretical SCF-CI 2-PyCOOH -243.0 ( 2.63 -383.4 ( 2.7 calculations. 3-PyCOOH -221.5 ( 1.518 -361.9 ( 1.7 4-PyCOOH -234.8 ( 4.73 -375.2 ( 4.8 14 Literature Cited 2-PyCOCH3 -41.3 ( 2.9 -181.7 ( 3.0 14 3-PyCOCH3 -35.2 ( 2.8 -175.6 ( 2.9 (1) Ribeiro da Silva, M. A. V.; Matos, M. A. R. Recent work on the 14 4-PyCOCH3 -36.2 ( 2.0 -176.6 ( 2.1 thermochemistry of some nitrogen heterocycles. Pure Appl. Chem. a 2-PyCOOCH3 -208.8 ( 1.8 -349.2 ( 1.9 1997, 69, 2295-2305. a 3-PyCOOCH3 -214.1 ( 2.0 -354.5 ( 2.1 (2) Ribeiro da Silva, M. A. V. Thermochemistry of substituted pyridines a 4-PyCOOCH3 -218.9 ( 1.8 -359.3 ( 1.9 and analogous heterocycles: the enthalpic increments for a group Bz 82.6 ( 0.734 additivity scheme. Pure Appl. Chem. 1999, 71, 1257-1265. BzCOOH -294.1 ( 2.234 -376.7 ( 2.3 (3) Ribeiro da Silva, M. D. M. C.; Matos, M. A. R.; Vaz, M. C.; Santos, 34 L. M. B. F.; Pilcher, G.; Acree, W. E., Jr.; Powell, J. R. Enthalpies of BzCOCH3 -86.7 ( 1.7 -169.3 ( 1.8 BzCOOCH -276.1 ( 4.035 -358.7 ( 4.1 combustion of the pyridine N-oxide derivatives: 4-methyl-, 3-cyano-, 3 4-cyano-, 3-hydroxy-, 2-carboxy-, 4-carboxy-, and 3-methyl-4-nitro, a and of the pyridine derivatives: 2-carboxy-, and 4-carboxy-. The This work. dissociation enthalpies of the N-O bonds. J. Chem. Thermodyn. 1998, 30, 869-878. Table 12. Enthalpic Increments for the Methylation of the Pyridine (4) Ribeiro da Silva, M. A. V.; Matos, M. A. R.; Rio, C. M. A. Standard Carboxylic Acid Isomers and of Benzoic Acid molar enthalpy of formation of 2,4,6-trimethylpyridine. J. Chem. - -CH3 Thermodyn. 1997, 29, 901 906. increment/ (5) Ribeiro da Silva, M. A. V.; Matos, M. A. R.; Amaral, M. L. P. F. ° ‚ -1 ‚ -1 compound ∆fHm(g)/kJ mol kJ mol Standard molar enthalpies of formation of some chloropyridines. J. - a 3 Chem. Thermodyn. 1997, 29, 1535 1543. 2-PyCOOCH3/2-PyCOOH -(208.8 ( 1.8) /-(243.0 ( 2.6) 34.2 ( 3.2 a 18 (6) Ribeiro da Silva, M. A. V.; Matos, M. A. R.; Rio, C. M. A.; Morais, 3-PyCOOCH3/3-PyCOOH -(214.1 ( 2.0) /-(221.5 ( 1.5) 7.4 ( 2.5 - ( a - ( 3 ( V. M. F. Thermochemical and theoretical studies of 4-methylbiphenyl, 4-PyCOOCH3/4-PyCOOH (218.9 1.8) / (234.8 4.7) 15.9 5.0 ′ ′ ′ - ( 35 - ( 34 ( 4,4 -dimethylbiphenyl, 4,4 -dimethyl-2,2 -bipyridine. J. Chem. Soc., BzCOOCH3/BzCOOH (276.1 4.0) / (294.1 2.2) 18.0 4.6 Faraday Trans. 1997, 93, 3061-3065. (7) Ribeiro da Silva, M. A. V.; Matos, M. A. R.; Rio, C. A.; Morais, V. a This work. M. F.; Wang, J.; Nichols, G.; Chickos, J. S. A Thermochemical and theoretical study of the phenylpyridine isomers. J. Phys. Chem. A 2000, in this table, was then calculated using the estimated crystalline 104, 1774-1778. (8) Ribeiro da Silva, M. D. M. C.; Gonc¸alves, J. M.; Acree, W. E., Jr. heat capacity for this compound. Standard molar enthalpy of sublimation of crystalline 3-pyridinecar- boxylic acid. J. Chem. Thermodyn. 2000, 32, 1071-1073. Conclusion (9) Ribeiro da Silva, M. D. M. C.; Gonc¸alves, J. M.; Ferreira, S. C. C.; da Silva, L. C. M.; Sottomayor, M. J.; Pilcher, G.; Acree, W. E., Jr.; The results for the enthalpies of formation reported in the Roy, L. E. Experimental thermochemical study of the enthalpies of present work for the methylpyridine carboxylate isomers are formation and sublimation of isonicotinamide, picolinamide, nicoti- listed in Table 11 together with the values available in literature namide, isonicotinamide N-oxide, and nicotinamide N-oxide. The dissociation enthalpies of the N-O bonds. J. Chem. Thermodyn. 2001, for the pyridine carboxylic acid and acetylpyridine isomers as 33, 1263-1275. well as the values for benzoic acid, acetylbenzene, and meth- (10) Morais, V. M. F.; Miranda, M. S.; Matos, M. A. R. Thermochemical ylbenzoate. This table also includes the enthalpic increments study of the ethylpyridine and ethylpyrazine isomers. Org. Biomol. )- Chem. 2003, 1, 4329-4334. for the substitution of the functional groups X COOH, (11) Gomes, J. R. B.; Amaral, L. M. P. F.; Ribeiro da Silva, M. A. V. -COCH3, and -COOCH3 on the pyridine (Py) and benzene Gas-phase thermochemistry of chloropyridines. Chem. Phys. Lett. (Bz) molecules. There is evidence of a similar trend and 2005, 406, 154-160. magnitude on the relative variations of the gaseous standard (12) Matos, M. A. R.; Morais, V. M. F.; Ribeiro da Silva, M. D. M. C.; Marques, M. C. F.; Sousa, E. A.; Castin˜eiras, J. P.; Santos, C. P.; molar enthalpies of formation of pyridine derivatives isomers Acree, W. E., Jr. Thermochemical and theoretical studies of dimeth- comparatively with the correspondent benzene derivatives. ylpyridine-2,6-dicarboxylate and pyridine-2,3-, pyridine-2,5-, and - The energetic contributions of the functional group -COOCH , pyridine-2,6-dicarboxylic acids. J. Chem. Eng. Data 2005, 50, 1184 3 1191. as meta or para substituent on the pyridine ring, compared with (13) Morais, V. M. F.; Miranda, M. S.; Matos, M. A. R. Experimental and the effects observed for identical substitutions of -COOH and computational thermochemistry of the dihydroxypyridine isomers. J. Chem. Thermodyn. 2006, 38, 450-454. -COCH3, show a parallel tendency for the different isomers; - (14) Freitas, V. L. S.; Oliveira, L. I. P.; Ribeiro da Silva, M. D. M. C. however, the effect of the COOCH3 substitution in ortho Standard molar enthalpies of formation of the acetylpyridine isomers. position reveals a relative small destabilization of the isomer J. Chem. Thermodyn. (in press). that is probably due to the steric hindrance of a bulky group (15) Trujillo, J. I.; Gopalan, A. S. Facile esterification of sulfonic acids located at a carbon adjacent to the heteroatom on the pyridine and carboxylic acids with triethylorthoacetate. Tetrahedron Lett. 1993, 34, 7355-7358. ring. Indeed, the extended π electron delocalization between (16) Catalogue Handbook of Fine Chemicals and Laboratory Equipment; the aromatic ring and the group -COOCH3, may be unfavorably The Sigma-Aldrich Chemical Co.: Gillingham, U.K., 2005-2006. affected by the interaction of the lone pair of electrons on the (17) Gundry, H. A.; Harrop, D.; Head, A. J.; Lewis, G. B. Thermodynamic properties of organic oxygen compounds. 21. Enthalpies of combustion nitrogen. of benzoic acid, pentan-1-ol, and hexadecane-1-ol. J. Chem. Thermo- From our work, it was also possible to calculate the enthalpic dyn. 1969, 1, 321-332. increments for the methylation of the pyridine carboxylic acid (18) Bickerton, J.; Pilcher, G.; Al-Takhin, G. Enthalpies of combustion of the three aminopyridines and the three cyanopyridines. J. Chem. isomers, which are summarized in Table 12, along with the Thermodyn. 1984, 16, 373-378. enthalpic increments for the methylation of benzoic acid. (19) Ribeiro da Silva, M. D. M. C.; Santos, L. M. N. B. F.; Silva, A. L. Recently, Roux et al.35 reported an enthalpic increment of (18.0 R.; Fernandes, O.; Acree, W. E., Jr. Energetics of 6-methoxyquinoline - ( 4.6) kJ‚mol-1 for the methylation of benzoic acid, which is and 6-methoxyquinoline N-oxide: the dissociation enthalpy of the (N O) bond. J. Chem. Thermodyn. 2003, 35, 1093-1100. in good agreement with the increment for the dimethylation of (20) Skinner, H. A.; Snelson, A. The of combustion of the four pyridine-2,6-dicarboxylic acid.12 The present results point to a isomeric butyl alcohols. Trans. Faraday Soc. 1960, 56, 1776-1783. Journal of Chemical and Engineering Data, Vol. 52, No. 2, 2007 585

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